Responses of the deep ocean carbonate system to carbon reorganization during the Last Glacial-interglacial cycle

We present new deep water carbonate ion concentration ([CO₃^2-]) records, reconstructed using Cibicidoides wuellerstorfi B/Ca, for one core from Caribbean Basin (water depth=3623m, sill depth=1.8km) and three cores located at 2.3-4.3km water depth from the equatorial Pacific Ocean during the Last Glacial-interglacial cycle. The pattern of deep water [CO₃^2-] in the Caribbean Basin roughly mirrors that of atmospheric CO₂, reflecting a dominant influence from preformed [CO₃^2-] in the North Atlantic Ocean. Compared to the amplitude of ~65μmol/kg in the deep Caribbean Basin, deep water [CO₃^2-] in the equatorial Pacific Ocean has varied by no more than ~15μmol/kg due to effective buffering of CaCO₃ on deep-sea pH in the Pacific Ocean. Our results suggest little change in the global mean deep ocean [CO₃^2-] between the Last Glacial Maximum (LGM) and the Late Holocene. The three records from the Pacific Ocean show long-term increases in [CO₃^2-] by ~7μmol/kg from Marine Isotope Stage (MIS) 5c to mid MIS 3, consistent with the response of the deep ocean carbonate system to a decline in neritic carbonate production associated with ~60m drop in sea-level (the "coral-reef" hypothesis). Superimposed upon the long-term trend, deep water [CO₃^2-] in the Pacific Ocean displays transient changes, which decouple with δ¹³C in the same cores, at the start and end of MIS 4. These changes in [CO₃^2-] and δ¹³C are consistent with what would be expected from vertical nutrient fractionation and carbonate compensation. The observed ~4μmol/kg [CO₃^2-] decline in the two Pacific cores at >3.4km water depth from MIS 3 to the LGM indicate further strengthening of deep ocean stratification, which contributed to the final step of atmospheric CO₂ drawdown during the last glaciation. The striking similarity between deep water [CO₃^2-] and ²³⁰Th-normalized CaCO₃ flux at two adjacent sites from the central equatorial Pacific Ocean provides convincing evidence that deep-sea carbonate dissolution dominantly controlled CaCO₃ preservation at these sites in the past. Our results offer new and quantitative constraints from deep ocean carbonate chemistry to understand roles of various mechanisms in atmospheric CO₂ changes over the Last Glacial-interglacial cycle.